Even with the volume turned all of the way up the CD player is loaded with 33K ohms and the Zout of your player is only 50 ohms, a ratio of about 700:1. Conventional wisdom says at least 10:1 with higher better and no benefit beyond 100:1. I think 10:1 is a bit low but you are in a very good place. I would be tempted to try a Placette or a transformer control with those ratios.
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